Lubricating grease comprising a synthetic oil and a complex thickener



2,750,341 LUBRICATING GREASE COMPRISING A SYN- rrmrrc 01L AND A CUMPLEX THICKENER Arnold .l. Morway, (Clark Township, Union County, David W. Young and Paul V. Smith, Jr., Westfield, N. J assignors to Esso Research and Engineering Comparty, a corporation of Delaware No Drawing. Application December 2% 1951, Serial No. 263,971 14 Claims. (Cl. 252-42) The present invention relates to improved lubricating greases and particularly to lubricating greases of improved stability at high temperatures and under adverse conditions. It relates more especially to greases using a non-hydrocarbon or synthetic oil as the liquid constituent and employing a complex thickener.

In the prior art various attempts have been made to formulate lubricating grease compositions of improved high temperature performance characteristics. Various types of mineral oils and synthetic oils have been combined with various thickeners. As a general rule, perior art lubricating greases have been made by adding thickeners to hydrocarbon oils or mineral base oils. The thickeners have usually consisted of the fatty acid soaps of the alkali and alkaline earth metals, though some nonsoap thickeners have been used. Recently some improvements in the thickeners have been made by incorporating low molecular weight salts or other modifiers into the soaps. Thus salts of acetic, acrylic, and other acids have been used. These low molecular weight constituents, when combined with the common fatty acid soaps of high molecular weight, have shown definite advantages in some cases in raising the dropping point or in stabilizing the grease against oil separation in moderately high temperature service.

Recently, the development of jet engines and turbojet for aircraft has accentuated the need for lubricants useful for service at still higher temperatures. Certain new types of synthetic oils have been proposed as lubricants in such engines and the present invention involves the use of these as ingredients in new types of lubricating greases. Difliculty has been encountered in preparing satisfactory greases from such oils, and another feature of this invention involves a successful method for such preparation.

Many of the synthetic oils such as dibasic acid esters, complex esters, e. g. of polybasic acids and glycols or polyglycols, and the like, cannot be thickened very satisfactorily with conventional soaps in the same manner as mineral oils. Some simple esters have been thickened to grease consistency with certain special soaps, e. g. lithium stearate, but the use of synthetic oil base greases has been very limited notwithstanding the very good lubricating properties which some of the synthetic materials, especially complex synthetic esters, possess.

An important aspect of the present invention is the discovery that certain complex soap-salt thickeners, e. g. a fatty acid soap and low molecular weight organic salt complex of lithium has outstanding properties as a thickener for some of the new types of synthetic materials which have been very difficult in the past to convert to lubricating greases.

Another aspect of the present invention is the discovery of a grease composition of exceptional stability and fine lubricating properties at high temperatures composed primarily of a complex type aliphatic ester thickened to grease consistency with a lithium soap-salt complex of the type mentioned above. The complex ester mentioned is preferably of the type prepared by esterifying dibasic acids and dihydric alcohols, adding a monovalent substituent to complete the esterification. One type of such a lubricant is prepared by esterifying a glycol 01 nite States Patent 2,750,341 Patented June 12, 1956 a thioglycol with adipic acid or seba'cic acid or one of their homologues or thio equivalents, to produce an ester having, for example, a dihydric alcohol residue at the center and two dibasic acid residues attached, one on either side of the center. The end groups of the acid residues are esterified with a monohydric alcohol. Alternatively, a dibasic acid center may be esterified with two mols of dihydric alcohol which in turn have their esterification completed by treatment with two mols of monobasic acid. Elsewhere in this specification the expression complex ester of dihydric alcohol and dibasic acid will be understood to refer to either of these types. The conversion of lubricating oils to greases by additions of a thickener is frequently a difficult matter. Most mineral base oils can be converted to greases by adding sodium or calcium soaps of the higher saturated fatty acids, e. g. sodium, stearate, calcium stearate, etc. For the simple synthetic ester oils, such as di-2-ethyl hexyl sebacate, for example, dry lithium stearate, previously prepared, is a reasonably good thickener, forming a grease having good lubircating qualities over a wide temperature range. In some oils, however, including some of the higher viscosity index types of mineral oils, even these soaps do not form stable colloids or greases. These properties depend somewhat on the solubility of the soap in the oil.

Simple soaps, e. g. the sodium or calcium soaps of C12 to C22 fatty acids, are entirely too soluble in the com.- plex esters described above to make a good grease structure. Furthermore, they cannot be prepared in situ in such esters because the basic saponifying agent, 6. g. metal oxide, hydroxide or carbonate, hydrolyzes or decomposes the ester.

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In the prior art it has been suggested that greases employing synthetic ester oils may be made by forming a soap in a small quantity of mineral oil and thereafter blending in the ester, e. g. di-Z-ethyl hexyl sebacate, or dialkyl esters of adipic acid, etc. This technique apparently can be employed with many of the esters where the soaps used are not too soluble, but even in these cases the superior viscosity index and high flash point or low volatility characteristics of the synthetic oil are at least partially lost. With complex esters of the type mentioned above, where these properties are very important, mineral oil dilution is most objectionable, aside from the fact that simple soaps are not effective thickeners.

According to the present invention, a complex ester as described above may be thickened by the use of 5 to 25% by weight, based on the total composition, of the complex soap thickener. This complex thickener may consist of various soaps, a preferred material being about 0.5 to 2 mols lithium soap of fatty acid in the C12 to C22 range, combined with about 1 mol of the lithium soap of a lower carboxylic acid, i. e., having a molecular weight below about 100. The low molecular weight salt is preferably the acetate, but may be a propionate or formate, or it may be an unsaturated material of similar molecular weight range such as an acrylate, methacrylate or crotonate. The formates are diflicult to disperse and higher acids than the propionate do not form good complexes. The saturated materials are preferred for the long chain fatty acids. For the low molecular weight acids, either saturated or unsaturated are about equally desirable. The most suitable fatty acids are of about C18 average chain length. The acetate or acrylate (the lower molecular weight salts) are preferred for the low molecular weight component of the complex which appears to be of the Werner type.

The specific preferred thickener for the complex ester greases of the present invention is composed approximately of one mol of the lithium soap of fatty acids of approximately the stearic acid chain length (C13) in combination with about one mol of lithium acetate. Instead of straight stearic acid, the fatty acids resulting from the hydrogenation of fish oils, e. g. Hydrofol Acids 54 are very satisfactory for making the soaps. These average about C18, though they may range from C12 to C22. The quantity of soap complex required may vary somewhat depending upon the desired consistency of the grease. in general, proportions will be not less than 5 nor more than of the Weight of the total composities as the complex esters but they do have excellent high temperature stability. A very satisfactory grease has been prepared using about half and half of the complex ester and silicone.

In order to prevent the hydrolysis of the ester, it is necessary to form the soap-salt complex separately. While this could be done in mineral oil, as noted above, the product of the present invention is best made by using a dry preformed soap complex. The latter is prepared by direct neutralization of a mixture of the fatty acid (stearic acid or hydrogenated fish oil acids) and low molecular weight acid (preferably acetic acid) with the base in an aqueous medium. Alkali metal bases are preferred, lithium being preferred and sodium next. Ordinarily the hydroxides are employed. Proportions of base should be suflicicnt to neutralize the high and low molecular weight acids Without substantial excess or deficiency, although a minor degree of free acidity or alkalinity may be tolerated.

Alternatively, the acetate, or other low molecular weight salt, and the soap may be formed separately, dried and ground into fine powders, and the powdered materials mixed together before adding to the synthetic oil. A soap-salt complex apparently is formed, much as in the procedure described in the preceding paragraph. The invention will be more fully understood by reference to the following specific examples.

Example I A complex ester oil of moderately high lubricating viscosity at ordinary temperatures, was prepared by coesterifying sebacic acid and glycol, using a monohydric alcohol to complete the esterification. One mol of glycol and two mols of dibasic acid were used, along with two mols of monohydric alcohol such as the commercial alcohol mixture derived from coconut oil and sold under the trade name Lorol B. 1 This ester was thickened to a grease consistency by using 15% of a one-to-one mol ratio lithium acetate-lithium soap complex. The soap was a hydrofolate, being derived from Hydrofol Acids 54, or commercial substantially saturated hydrogenated fish oil acids which have the general average chain length and chemical properties of stearic acid. The dry soap-salt compound or complex and the complex ester were m xed together and heated with continuous stirring to a temperature of about 500 F, or until all soap-salt complex had melted. The molten grease was then cooled without stirring. It had a smooth buttery appearance, with an unworked penetration value of 200 mm./l0 in the ASTM penetrometer. When worked 60 strokes in the standard ASTM grease worker, it showed a penetration value of 220 mm./ 10. It was still stable after working 100,000 strokes in the grease worker. Its dropping point was 408 F. and it showed no water solubility.

instead of using the complex ester described above as the only oil ingredient, a part of such ester may be replaced by an ordinary simple diester of dibasic acid such as di-Z-ethylhexyl sebacate or adipate. in any case, however, the latter simple ester should not exceed in quantity the amount of complex ester in the oil blend. It is desirable also to add an oxidation inhibitor, for example 1% by weight, based on the total composition, of phenothiazine. A grease prepared as just described was found to have a dropping point of 440 F., a worked penetration of 200 mm./ 10 after 60 strokes, complete stability after l00,000 strokes, and just a bare trace of water solubility.

Example II A grease was prepared using 14.3% by weight of lithium stearate, 14.3% of di-2-ethylhexyl sebacate, 28.6% of a complex ester derived from two mols of adipic acid, one mol of triethylene glycol, and two mols of ethyl hexanol, and 42.8% of a silicone oil, specifically a fairly viscous polymethyl phenylsiloxane, and 1% of phenothiazine. The mixtures of lithium soap and simple diester were first heated to 350 F. while stirring. The complex ester was then added and the grease was heated further to 400 F. Thereafter the product was allowed to cool in thin layers. The silicone oil was blended into the cool grease and the product was homogenized under conditions of high shear rates. The product had a smooth buttery appearance, a dropping point of 390 F no measurable solubility in boiling water, an unworked penetration of 285 mm./ 10 at the standard test temperature of 77 F. and a worked penetration after 60 strokes that was unchanged, 285 mm./ 10. After working 100,000 strokes, the penetration number increased only slightly to 305 mm./ 10. The product was tested also for its resistance to oxidation with the following results:

Norma Hoifmann bomb oxidation test:

p. s. i. initial oxygen pressure 210 F. Pressure drop (oxygen)hours to a 5 p. s. i.

drop in pressure 584+ Another grease was prepared using 16% by weight of lithium stearate alone (without low molecular weight salt), 41.5% of an ester oil mixture which contained 35% of di-2-ethylhexyl sebacate and 65% of a complex ester derived from polyethylene glycol of about 500 molecular weight, sebacic acid and ethyl hexanol. The product was prepared as above and 41.5% of a dimethyl silicone polymer oil (Silicone Fluid 710 as marketed by the Dow- Corning Corporation) was added, along with 1% phenylalpha-naphthlyamine oxidation inhibitor. This grease was smooth and buttery in appearance with a dropping point of 385 F. Its unworked penetration was 265 mm./ 10 under standard conditions, 77 F. It showed a worked penetration of 270 mm./ 10 after 60 strokes and 290 mm./ 10 after 100,000 strokes. This grease was relatively very soft for its high soap content, due to the excessive solubility of the soap in the complex ester. It is found that at least half of the oil should be complex ester. If more silicone oil is used the soap-salt complex separates from the oil.

Example IV Instead of using lithium stearate, a complex soap was prepared as the thickening agent using 66.2% of substantially saturated fatty acids of about C18 average chain length sold under the trade name Hydrofol Acids 53 which are hydrogenated fish oil acids. To these acids were added 13.3% by Weight based on the total soap composition, of glacial acetic acid and 20.5% of lithium hydroxide monohydrate. The fatty acid was melted in water at 150 F., and the glacial acetic acid was added thereto. The mixture was neutralized with a slight excess of the lithium hydroxide dissolved in boiling water. The soap was dried and powdered.

Using the above complex soap in proportions of 15% there were added 1% phenothiazine, 42% of a complex ester derived from one mol adipic acid, one mol ethyl hexanoic acid, one mol of triethylene glycol, and one mol of 2-ethyl hexanol. To this was added 42% of the silicone oil 710. The ingriedents were all mixed together while heating to 450 F., and then the product was allowed to cool. It formed an excellent grease of smooth texture and almost transparent. Its dropping point was 410 F. and its unworked penetration 200 mm./ at 77 F. Its worked penetration was 200 mm./ 10 after 60 strokes and 210 mm./ 10 after 100,000 strokes. It had no measurable water solubility. It was a much firmer grease than that of Example III with a somewhat lower soap concentration.

Example V A product was made as in Example IV, except that di-2-ethylhexyl sebacate was used as the oily ester instead of the complex ester of Example IV. It was found that the soap became preferentially dispersed in the diester forming a hard coke-like material and leaving the silicone oil quite separate and unthickened. No grease product of smooth consistency could be prepared, the diester by itself being quite unsuitable for the purposes of this invention. On the other hand the product of Example IV formed an excellent grease of good structural stability. The product was smooth and hard and it required no homogenization. On the other hand the products of Examples II and HI had some disadvantages. The product of Example II had a good grease structure but required mechanical homogenization. It showed a tendency also to become crystalline at high temperatures. The product of Example III became semi-fluid upon storage and showed no structural stability. Apparently it is necessary to use a complex soap-salt in this type of liquid thicken- It will be understood that some variations can be made in proportions of ingredients and that various synthetic esters may be substituted for those described above. In general, however, the complex ester should form a very substantial part of the liquid lubricant material. The silicone oils may be used for a part, but preferably not for a very major part of the liquid portion. Conventional modifiers such as antioxidants of various types, tackiness agents, metal deactivators, plasticizers, extreme pressure agents and the like, may be incorporated in the usual proportion as will be apparent to those skilled 1n the art.

What is claimed is:

1. A lubricating grease composition comprising a major proportion of a substantially completely esterified complex dibasic acid-dihydric alcohol-monovalent substituent aliphatic ester oil base of lubricating grade and viscosity, said dibasic acid being selected from the group consisting of sebacic and adipic acid, said dihydnc alcohol being triethylene glycol and said monovalent substituent being selected from the group consisting of ethyl hexanoic acid, ethyl hexanol and a mixture of C10 to C18 alcohols having an average of 13.5 carbon atoms, and 5 to 25% by weight, based on the total composition, of a preformed complex soap thickener consisting essentially of alkali metal soap of fatty acid in the C10 to C22 range and alkali metal salt of a carboxylic acid having a molecular weight of below about 100, the molar ratio of said fatty acid soap to said carboxylic acid salt being in the range of about 0.5 :1 to 2:1.

2. Composition according to claim 1 wherein the oil base is composed substantially entirely of the complex ester.

3. Composition according to claim 1 wherein up to about one-half of the base oil is a silicone oil of lubricating viscosity.

4. Composition according to claim 1 wherein the oil base comprises substantially equal proportions of complex ester and silicone oil.

5. Composition according to claim 1 wherein said alkali metal is lithium.

6. Composition according to claim 1 wherein said alkali metal is sodium.

7. Composition according to claim 1 wherein the soap and salt. are present in about equimolar proportions.

8. Composition according to claim 1 wherein the salt is lithium acetate.

9. A lubricating grease composition consisting essentially of a substantially completely esterified dibasic aciddihydric alcohol-monovalent substituent aliphatic ester oil of lubricating grade and viscosity, said dibasic acid being selected from the group consisting of sebacic acid and adipic acid, said dihydric alcohol being triethylene glycol and said monovalent substituent being selected from the group consisting of ethyl hexanoic acid, ethyl hexanol and a mixture of C10 to C12 alcohols having an average of 13.5 carbon atoms, and 5 to 25% by weight, based on the total composition, of a lithium soap of saturated fatty acids in the C12 to C22 range complexed with lithium salt of carboxylic acids, of the C1 to C3 range selected from the group consisting of formic acid, acetic acid, propionic acid and acrylic acid.

10. Composition according to claim 9 wherein the soap and salt are present in substantially equimolar proportions.

11. Composition according to claim 9 wherein the salt is of saturated carboxylic acid.

12. Composition according to claim 9 wherein the salt is an acetate.

13. Composition according to claim 9 wherein the soap is predominantly of an average of C18 fatty acid.

14. A lubricating grease composition consisting essentially of a mixed base vehicle of a substantially completely esterified dibasic acid-dihydric alcohol-monovalent substituent aliphatic ester oil of lubricating grade and viscosity, said dibasic acid being selected from the group consisting of sebacic acid and adipic acid, said dihydric alcohol being triethylene glycol and said monovalent substituent being selected from the group consisting of ethyl hexanoic acid, ethyl hexanol and a mixture of C10 to C18 alcohols having an average of 13.5 carbon atoms, and a dimethyl silicone polymer oil of lubricating viscosity, said silicone oil amounting to not more than onehalf of said mixed base vehicle, said vehicle being thickened to a grease consistency with about 15 by weight, based on the total composition, of a preformed complex of about 1 mol of lithium soap of saturated fatty acids in the C12 to C22 range and about 1 mol of lithium acetate.

References Cited in the file of this patent UNITED STATES PATENTS 2,455,892 Fraser Dec. 7, 1948 2,508,741 Ashburn et a1 May 23, 1950 2,551,931 Currie May 8, 1951 2,575,195 Smith Nov. 13, 1951 2,575,196 Smith Nov. 13, 1951 2,581,127 Morway et al. Jan. 1, 1952 2,586,693 Morway et a1 Feb. 19, 1952 2,607,735 Sproule et al Aug. 19, 1952 2,628,974 Sanderson Feb. 17, 1953 2,639,266 Dilworth et al. May 19, 1953 

1. A LUBRICATING GREASE COMPOSITION COMPRISING A MAJOR PROPORTION OF A SUBSTANTIALLY COMPLETELY ESTERIFIED COMPLEX DIBASIC ACID-DIHYDRIC ALCOHOL-MONOVALENT SUBSTITUENT ALIPHATIC ESTER OIL BASE OF LUBRICATING GRADE AND VISCOSITY, SAID DIBASIC ACID BEING SELECTED FROM THE GROUP CONSISTING OF SEBACIC AND ADIPIC ACID, SAID DIHYDRIC ALCOHOL BEING TRIETHYLENE GLYCOL AND SAID MONOVALENT SUBSTITUENT BEING SELECTED FROM THE GROUP CONSISTING OF ETHYL HEXANOIC ACID, HEXANOL AND A MIXTURE OF C10 TO C18 ALCOHOLS HAVING AN AVERAGE OF 13.5 CARBON ATOMS, AND 5 TO 25% BY WEIGHT, BASED ON THE TOTAL COMPOSITION, OF A PREFORMED COMPLEX SOAP THICKENER CONSISTING ESSENTIALLY OF ALKALI METAL SOAP OF FATTY ACID IN THE C10 TO C22 RANGE AND ALKALI METAL SALT OF A CARBOXYLIC ACID HAVING A MOLECULAR WEIGHT OF BELOW ABOUT 100, THE MOLAR RATIO OF SAID FATTY ACID SOAP TO SAID CARBOXYLIC ACID SALT BEING IN THE RANGE OF ABOUT 0.5:1 TO 2:1. 